Very useful tool Rated 4 out of 5 stars
I frequently use this tool in addition to the W3C validator. Both tools analyze the pages slightly different, and the combination of both gives a more complete impression. The best thing about Total Validator is the possibility to take screen shots. This allows me to check compatibility with IE6, a browser, which is still used too much but which I don't want to install.
Unfortunately, Total Validator does not support MathML.